1. Field of the Invention
The present invention relates to the field of scanning. More specifically, the present invention relates to a method of scanning an image on an original, and a scanning device arranged to perform the method of scanning.
In a scanning device of the present invention, an optical sensor is provided comprising sensor elements for each of a number of basic colors to generate pixel values of a scanned image. Each pixel value represents an optical density of a pixel of the image. In this way, a color image of the original is obtained using a color scanner. The image on the original may be a grey scale or B/W (black and white) image, or may be a color image.
2. Background of the Invention
A scanner usually has a light source emitting white light. In reduction type scanners, an optical sensor with one or more linear CCD (Charge Coupled Device) arrays and a lens are arranged to focus lines of an original onto the CCD line arrays. In contact type scanners, lines of the original are captured onto the CCD line arrays through a lens array.
In a color scanner, there are, for example, at least three CCD line arrays having color filters for red (R), green (G), and blue (B) light, respectively, thus forming sensor elements for the basic colors of red, green, and blue light. Typically, the sensor elements for different colors have different light sensitivities. When a color image is to be obtained from an original, the original is scanned, and the signals from the sensor elements for red, green, and blue light are mathematically mixed or processed in order to calculate the intensities from the RGB values of each pixel of the image. Hence, the color level of a pixel is a function (e.g. by applying an algebraic scheme or by applying look-up tables) of the RGB values, wherein the function is predefined or otherwise determined to obtain a desired image.
Usually, each pixel of the original image is scanned by each of the sensor elements for different colors. Due to a mechanical spacing between the sensor elements or CCD line arrays, each pixel is scanned by time-multiplexing signals originating from different sensor elements. An output image is composed from said signals by giving each signal an appropriate delay.
The scan speed of a reduction type or a contact type scanner is limited by the maximum clock speed of the CCD. This is a serious limitation in view of a demand for high scan resolutions to be obtained in a shortest possible time.